Antireflective films are used for prevention of reflected background and improvement of viewability in image display devices, such as a cathode-ray tube (CRT), a plasma display panel (PDP), an electroluminescent display (ELD) and a liquid crystal display (LCD).
In order to avoid a contrast drop by reflections and reflected images, an antireflective film is placed at the outermost surface of a display so that the reflectance is reduced utilizing the principle of optical interference. Therefore, such a film carries a high risk of scratches, and it is an important problem to confer excellent scratch resistance upon the film.
Such an antireflective film can be made by forming a low refractive index layer having a proper thickness as the uppermost layer over a support (substrate), and further between them a high refractive index layer, an intermediate refractive index layer and a hard coating layer when required. For achievement of a low reflectivity, it is appropriate that a material having the lowest possible refractive index be used in the low refractive index layer. In addition, the antireflective film is required to have high scratch resistance because it is used at the outermost surface of a display. In order to achieve high scratch resistance in a thin film having a thickness in the neighborhood of 100 nm, the film in itself is required to have sufficient strength and adhesion to a lower layer.
For lowering the refractive index of a material, a method of introducing a fluorine atom into the material or a method of reducing the density of the material (by introduction of pores) can be adopted. However, these methods have tendencies to impair film strength and adhesiveness, and thereby to lower scratch resistance. So compatibility between low refractive index and high scratch resistance has been a difficult problem.
JP-A-11-189621, JP-A-11-228631 and JP-A-2000-313709 describe the methods of improving scratch resistance by introducing polysiloxane structures into fluorine-containing polymers to lower the friction coefficients of film surfaces. Those methods each can have a measure of effect upon improvement of scratch resistance, but the use of them alone was unsuccessful at imparting satisfactory scratch resistance to films intrinsically lacking in film strength and interfacial adhesion.
On the other hand, JP-A-2002-156508 describes hardness elevation by curing radiation-curable resins under an atmosphere lowered in oxygen concentration. However, it was impossible to attain hardness satisfying for efficient manufacturing of an antireflective film in web form since there was a limitation to the concentration of oxygen replaceable with nitrogen in such a system.
JP-A-11-268240, JP-A-60-90762, JP-A-59-112870, JP-A-4-301456, JP-A-3-67697 and JP-A-2003-300215 describe the specific systems for nitrogen replacement. However, those systems required large quantities of nitrogen for the oxygen concentration therein to be lowered to the extent that a thin film such as a low refractive index layer was cured well, so they had a problem of a rise in manufacturing cost.
JP-B-7-51641 describes the method of irradiating a film substrate having thereon an ionizing radiation-curable resin coating with ionizing radiation while winding the film around a heating roll. However, this curing method was also insufficient to fully cure a special thin film such as a low refractive index layer.
Further, JP-A-2001-222105 describes the increase in photosensitivity of a light- and heat-sensitive recording material having a thickness of 0.1 to 50 μm by the combined use of an organoborate compound, a cationic compound capable of interacting with the organoborate compound and two or more specified compounds. However, there was some question as to whether or not the foregoing combined use was successful in curing a thin film having a thickness of the order of 100 nm, such as that of a constituent layer of an antireflective film, to the extent that the thin film cured could ensure sufficient hardness for use as an antireflective film.